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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Fri, 11 Jun 2021 15:11:48 +0200
Message-ID: <CAJfpegvYeNjWKc_9Aah+8NMicTus1CqqwQqLGUeYiQSKN9=gmQ@mail.gmail.com>
Subject: Re: [PATCH v6 5/7] kernfs: use i_lock to protect concurrent inode updates
To:     Ian Kent <raven@themaw.net>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>, Eric Sandeen <sandeen@sandeen.net>,
        Fox Chen <foxhlchen@gmail.com>,
        Brice Goglin <brice.goglin@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Rick Lindsley <ricklind@linux.vnet.ibm.com>,
        David Howells <dhowells@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Carlos Maiolino <cmaiolino@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, 9 Jun 2021 at 10:52, Ian Kent <raven@themaw.net> wrote:
>
> The inode operations .permission() and .getattr() use the kernfs node
> write lock but all that's needed is to keep the rb tree stable while
> updating the inode attributes as well as protecting the update itself
> against concurrent changes.
>
> And .permission() is called frequently during path walks and can cause
> quite a bit of contention between kernfs node operations and path
> walks when the number of concurrent walks is high.
>
> To change kernfs_iop_getattr() and kernfs_iop_permission() to take
> the rw sem read lock instead of the write lock an additional lock is
> needed to protect against multiple processes concurrently updating
> the inode attributes and link count in kernfs_refresh_inode().
>
> The inode i_lock seems like the sensible thing to use to protect these
> inode attribute updates so use it in kernfs_refresh_inode().
>
> The last hunk in the patch, applied to kernfs_fill_super(), is possibly
> not needed but taking the lock was present originally and I prefer to
> continue to take it so the rb tree is held stable during the call to
> kernfs_refresh_inode() made by kernfs_get_inode().
>
> Signed-off-by: Ian Kent <raven@themaw.net>

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>